The present invention relates to a precharge buffer. In particular, it relates to an open loop buffer design that allows a sampling load to be precharged to an output signal based on an input signal that is at or near either extreme of the rail voltage.
Purely transistor based open loop buffers do not provide good representation of input signals that have a value at or near the positive and negative voltage supplies, which are called the rail voltages, of an amplifier. The configuration of a class AB amplifier is well known as comprising a push-pull stage. The classic class AB push-pull stage comprises a PMOS transistor and an NMOS transistor having a common input to receive an input signal. The push-pull stage is biased by other level shifting transistors which themselves are biased off current sources or resistors. The bias stage therefore cannot accept inputs at the supply extremes due to constraints of the electrical components, such as the transistors and biasing sources. Therefore, the output from a class AB amplifier cannot transition accurately between the positive rail voltage and the negative rail voltage.
Accordingly, there is a need for an amplifier precharger that is open loop (and, therefore, fast responding), provides rail-to-rail voltages, is low noise, low power and has a high input impedance. Such an amplifier precharger would provide for high input impedance sampling that allows for precise sampling of voltages that are at the extremes of the analog power supply rails, in other words, precise sampling of voltages from rail to rail.